Write something. Write anything.
When you’re staring at a blank piece of paper and you don’t know what to write the best thing to do is to just start writing. Write something. Write anything. If you sit there staring waiting for the perfect word or the most stunning introduction you will soon get frustrated. What you initially produce may not be your usual best effort but at least you can always make changes and improvements.
If you write nothing then you still have nothing. Nothing that you can edit. No words that you can switch around or shorten or lengthen or replace. A bad effort is better than no effort. It is at least one step forward that can lead to other steps toward creating a satisfactory piece of writing.
Step away
The next step is to put away that initial effort for a little while. Go and get occupied and do something else. You could bake a cake or do your laundry or take a walk. When you are not being anxious over the piece you wrote you will find that additional ideas will come to your mind. Suddenly there is another thought you could develop or another perspective you had not even considered. When these come to mind jot them down. If you are out walking take a note in the Keep app on the smartphone that you always have with you. Ask someone to read your writing. Ask someone who will be honest and who can give you guidance. Even if they can only identify your mistakes in spelling and grammar that will still be helpful. Listen to their feedback and suggestions. This will guide you towards areas you should improve.
Shorten, lengthen or replace
Take the ideas you have collected and go visit your piece of writing like an old friend. Add your new ideas. Expand your perspective. You can even start all over and do a new piece now that you have the confidence, comfort and familiarity with your writing. What you wrote before can be the foundation or even inspiration for continuing and improving your writing. By now you probably have the idea for that arresting introduction and your words will be flowing on to the page. So out of that not very accomplished initial effort you will be creating a more polished product.
Edit your work. If you have your own deadline you could leave your writing alone for a little while and come back to it with fresh eyes. Not all good writing is produced from a single burst of speed and effort. Sometimes you have to turn to your piece again and again.
But just get started and you will see that you will beat that blank paper.